Aromatic dicarboxylic acids are important items of commerce. While they have varied uses as chemical intermediates, probably their largest use are as monomers for use in polymerizations, particularly to make thermoplastic polyesters and polyamides. For this and other purposes, very pure materials are, generally, required and new methods of purifying such aromatic dicarboxylic acids are constantly being sought.
Aromatic dicarboxylic acids such as phthalic acid, isophthalic acid and terephthalic acid may be prepared by an oxidation process in which mixed alkyl aromatics or particular xylenes are subjected to an oxidation reaction with heavy metal salts and bromine acting as catalysts. For example, paraxylene may be oxidized to terephthalic acid in the liquid phase with a catalyst consisting of cobalt acetate and hydrogen bromide dissolved in acetic acid. An oxygen containing gas such as air is injected into the reactor which is maintained at an elevated temperature of around 200.degree. C. and superatmospheric pressures of about 10 atmospheres. The amount of air which is injected into the reactor is sufficient to maintain an excess of oxygen in the off-gas over the stoichiometric requirement. The liquid which is drawn from the reactor will contain about 25 to about 35 percent solids. These solids can contain the desired terephthalic acid as well as contaminants or impurities in the form of the monocarboxylic acid, aldehydes, unoxidized materials, over oxidized materials, catalyst residues, etc. Most or all of these contaminants or impurities may affect the usefulness of the aromatic dicarboxylic acid as a monomer for polymerizing polyesters and polyamides.
Japanese Patent Application 51-52163 describes a process for the purification of 2,6-naphthalene dicarboxylic acid by formation and purification of its ammonium salt. There is no mention of using aliphatic diamines or recovery of useful byproduct salts in the process.
It is an object of the present invention to provide a convenient, efficient and effective process for making purified aromatic dicarboxylic acid from relatively impure aromatic dicarboxylic acid. The process described herein results in byproducts which are useful in the manufacture of nylons.